Over the years I have tried a number of different methods but one method stands out head and shoulders above the others. Take a length of PVC pipe of the same diameter you are already using and cut it lengthwise. You will now have two half circle lengths of PVC pipes. You will also need a supply of short screw in PVC roofing screws.
First step is to hold the half circle PVC lengths against the PVC structure with both curves facing the same way. Next drill attaching holes through both bits of PVC such that the diameter of the holes is the same as the inside diameter of your screws. Next create a sandwich of PVC pipe, then netting, then half circle PVC. Finally screw the sandwich together through the holes you previously drilled.
This construction is bug proof, storm proof and can easily be changed with the seasons. For example you could replace the netting with plastic sheeting during the winter. It even makes it possible to create a double glazing effect by repeating the exercise on the inside.
Hope you can follow these instructions and that you find it of use.